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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesThe visible ledge inside a window is usually the interior stool; the trim beneath it is the apron. Outside, the window has an exterior sill. Below the installed window, a concealed sill pan or flashing helps manage water. These parts sit near one another but serve different purposes.
Which parts are at the bottom of a window?
| Component | Where it is | What it does | Visible after installation? |
|---|---|---|---|
| Exterior sill | At the bottom of the window on the outside | Forms the exterior lower edge and is angled to drain water from its surface, according to the City of Detroit’s illustrated window guide. | Yes, from outside |
| Interior stool | At the base of the window on the inside | Provides the indoor ledge beneath the window; it is also commonly called the interior sill. See the Detroit guide and Andersen’s window glossary. | Yes, from inside |
| Apron | On the wall immediately below the interior stool | Finishes the area beneath the stool; it is interior trim, not a water barrier. See the Detroit guide and Andersen’s glossary. | Yes, from inside |
| Sill or pan flashing | At the bottom of the rough opening, beneath the installed window | Manages water at the opening and directs it away from the building. The U.S. Department of Energy’s Building Science Education guidance describes a back dam and flashing materials. | Usually concealed |
| Weep holes | Drainage openings in the window frame | Allow moisture that gets behind the glass to exit. The Building America Solution Center advises not to block them. | Often visible from outside |
Interior stool versus exterior sill
People often call the indoor ledge a “window sill.” In more precise terminology, the indoor piece is the stool, while the exterior sill is the lower part of the window on the outside. The Detroit guide defines the stool as the interior piece at the lower sash rail; Andersen’s glossary also distinguishes the stool beneath the window from the apron below it. To avoid confusion, specify interior stool or exterior sill when describing a repair or asking a contractor about a component.
What the apron does—and does not do
The apron is the visible finish trim beneath the interior stool. It gives the lower part of the window opening a finished appearance. It is not the exterior sill and should not be mistaken for the concealed flashing that manages water at the rough opening.
How sill flashing manages water
The bottom of a window opening is a vulnerable place for water entry. Proper sill flashing helps direct water away from the opening rather than allowing it to collect in the wall assembly. The Department of Energy describes a back dam and a formable, self-adhered membrane that can bend around the sill corners. Other options it identifies include non-elastic membrane, rigid manufactured pan flashing, and liquid flashing. The appropriate detail depends on the window and wall assembly, so follow the window manufacturer’s specifications and applicable installation guidance.
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- Slope to exterior 0°- 6°
- Works on sloped or flat sills
- Install with every window
- Superior protection against leaks
Keep drainage paths clear
Do not seal or obstruct the window’s weep holes. They provide an exit for moisture that may get behind the glass; blocking them can interfere with drainage. The Building America Solution Center also advises checking for pan or sill flashing at the bottom of window openings.
Installation alignment matters
For window installation, the Building America Solution Center calls for the unit to be plumb, level, and square, and for the manufacturer’s specifications to be followed. Those are general recommendations; the exact flashing and installation details must suit the specific window and wall assembly.
Quick Recap
Best Value
- Slope to exterior 0°- 6°
- Works on sloped or flat sills
- Install with every window
- Superior protection against leaks
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- Adjustable sill pan flashing prevents water damage from leaks.
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Rank #3
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- Reliable Rough Opening Sealing: ZIP System Flashing Tape for windows, doors, and rough openings supports seam sealing to help block moisture intrusion; 3" width is best for seam sealing
- Advanced Acrylic Adhesive: Advanced acrylic adhesive delivers superior bonding and long-duration bond strength for demanding construction tape applications and durable sealing
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